Gas turbine engines operate by passing a volume of high energy gases through a plurality of stages of vanes and blades, each having an airfoil, in order to drive turbines to produce rotational shaft power. The shaft power is used to drive a compressor to provide compressed air to a combustion process to generate the high energy gases. Additionally, the shaft power is used to drive a generator for producing electricity. In order to produce gases having sufficient energy to drive the compressor or generator, it is necessary to combust the air at elevated temperatures and to compress the air to elevated pressures, which again increases the temperature. Thus, the vanes and blades are subjected to extremely high temperatures, often times exceeding the melting point of the alloys comprising the airfoils.
In order to maintain the airfoils at temperatures below their melting point it is necessary to, among other things, cool the airfoils with a supply of relatively cooler bypass air, typically bleed from the compressor. The bypass cooling air is directed into the blade or vane to provide impingement and film cooling of the airfoil. Specifically, the bypass air is passed into the interior of the airfoil to remove heat from the alloy, and subsequently discharged through cooling holes to pass over the outer surface of the airfoil to prevent the hot gases from contacting the vane or blade directly. Various cooling air patterns and systems have been developed to ensure sufficient cooling of the leading edges of blades and vanes.
Typically, each airfoil includes a plurality of interior cooling channels that extend through the airfoil and receive the cooling air. The cooling channels typically extend through the airfoil from the inner diameter end to the outer diameter end such that the air passes out of the airfoil. In other embodiments, a serpentine cooling channel winds axially through the airfoil. Cooling holes are placed along the leading edge, trailing edge, pressure side and suction side of the airfoil to direct the interior cooling air out to the exterior surface of the airfoil for film cooling. The leading edge is subject to particularly intensive heating due to the head-on impingement of high energy gases. The head-on impingement may result in stagnation of air at the leading edge, increasing the mixing out of cooling air from leading edge cooling holes. In order to improve cooling effectiveness at the leading edge, a trench has been positioned at the leading edge in various prior art designs, such as disclosed in U.S. Pat. No. 6,050,777 to Tabbita et al., which is assigned to United Technologies Corporation. The trench allows the cooling air to spread radially before mixing with the turbine gases and eventually spreading out over the outer surfaces of the airfoil. There is a continuing need to improve cooling of turbine airfoil leading edges to increase the temperature to which the airfoils can be exposed to increase the efficiency of the gas turbine engine.